In diagnosis using a nuclear magnetic resonance, a required accuracy in a magnetic intensity of the magnet system is such that variation of one millionth in magnetic intensity is considered to be a problem because a magnetic intensity corresponds to a diagnosis place. There are three types of magnetic fields in MRI apparatuses. That is:
(1) A magnetic field that is a constant in time base and uniform in space, and has an intensity of generally more than 0.1 to several teslas and a variation range of about several ppm within a space for imagining (a space of a sphere or an ellipsoid with a diameter of 30 to 40 cm);(2) A magnetic field varying with a time constant of about one second or shorter and inclined in a space; and(3) A magnetic field caused by a high frequency wave having a frequency (several MHz or higher) corresponding to the nuclear magnetic resonance.
Out of them, the magnetic field of (1) is required to be constant in time base and spatially have homogeneity in the magnetic intensity with an extremely high accuracy in the region where a tomographic imaging of a human body is done. “High accuracy” means that an accuracy with an order of one millionth, such as ±1.5 ppm, in an imaging space FOV (Field View) with a diameter of, for example, 40 cm. A magnetic field distribution of which homogeneity is required to be extremely high, requires adjustment for a magnetic field after production and excitation of a magnet. Generally, an error in magnetic field in production is 1000 times or more greater than the permissible error margin of the magnetic field demanded for a uniform magnetic field. Magnetic field adjustment (shimming) required when the apparatus is installed after production requires a magnetic field adjustment apparatus and a method with an extremely high accuracy because an error in magnetic field is reduced from hundreds ppm to several ppm.
There is a conventional method of shimming using a linear programming. For example, there is a method described by the patent document 1 or the patent document 2 and applied to actual apparatuses for adjustment. However, the linear programming has the following problems.
(1) The liner programming requires a long time period for calculation to conduct accurate calculations of the magnetic field.
(2) The linear programming requires such an accuracy that a magnetic field with a high accuracy is controlled in accordance with setting and variation of each iron piece and current.
(3) When an erroneous shimming operation is conducted, it is difficult to specify the place where the erroneous shimming operation is done, so that restoration requires a lot of work.
In addition, a problem occurs duet to adjusting the magnetic field distribution with spherical harmonic functions as shown in FIG. 2. FIG. 2 is a chart showing an example of a conventional magnetic field adjustment method using the spherical harmonic functions (Patent Document 1).
The spherical harmonic functions are orthogonal on a spherical surface to form a base, but when a magnetic field with a spherical function distribution having a high accuracy is tried to generate, a fine adjustment for a magnetic adjustment mechanism is required because there is a mutual interference in the magnetic field adjustment mechanism and on a magnetic field evaluation surface of an aspheric surface. For example, a homogeneous magnetic field distribution is a distribution having the lowest-numbered spherical harmonic functions. However, it is impossible to actually generate this distribution accurately unless using a magnetic adjustment mechanism which perfectly encloses a magnetic adjustment region. Accordingly, the MRI of the prior art has no such a magnetic adjustment mechanism.